Actually an isolation primary coil may be physically the same as a Lay up/Step Plume transformer. The main difference is in the way they are used. Another difference is that in a normal transformer there will be capacitance between the 2 windings, between the windings and core, between the core and shell, etc. These capacitances bring in high incidence noise from outside, which will be transmitted in the secondary circuit. So [in an isolation transformer] wires are connected between each component, (not direct contact, but with insulation present). This permits a run current and eliminates unnecessary capacitance. Answer Iodise could make a start to answer this question. Isolation transformer: is a transformer with couplet separate windings, the primary and the substitute. There is an electrical isolation between the primary and the indirect. Nearly any type of primary winding with two non-connected windings could be considered an "isolation" transformer, in that it electrically isolates the primary voltage from the secondary voltage. There are also transformers with one winding (called auto-transformers) and connections for input and output. If the input is low resting potential and the output a higher, point in time you stool say it is a STEP-UP primary. If the input is high and the output is low, then you can say it is a STEP-DOWN. This type of transformer can not by used where harborage is a large factor.